If you want a good framework for understanding the composition & mechanics of why this is happening - do a quick google search of "Mike Green Passive Flows".
TLDR: SP500 + auto-enrolling 401ks + buying the index without valuation = That scene in The Sorcerer's Apprentice where the brooms won't stop filling cauldron.
TLDR: SP500 + auto-enrolling 401ks + buying the index without valuation = That scene in The Sorcerer's Apprentice where the brooms won't stop filling cauldron.